SQL & PL/SQL

Hi, Everyone,

I am trying to find out the difference in days between two dates and the query i am passing is

SELECT TO_char(sysdate, 'dd/mm/yyyy')-TO_char('15/11/2011', 'dd/mm/yyyy') DAYS FROM DUAL

the error i am getting is

ORA-01722: invalid number
01722. 00000 - "invalid number"
*Cause:
*Action:

Could anybody please help.

Thanks in advance